Lights going out

Yesterday I attended my first of twelve Physical Therapy sessions. I messed up my knee a few weeks ago, and because I am no spring chicken, there are consequences for walking wrong. First and foremost, there are a lot of balls in the room, it has a bit of the romper and that connotes playful, but by the looks on the faces of the other clients who were there before me, it's anything but.
My therapist Bill, liked my "PT" humor, so we were good, but then he started to bend my leg and I no longer gave a fuck about what he thought of me, I just wanted to kick him, but it wouldn't be a hard kick, because;"my resistance is weak" and I'm "not flexible". I did actually leave there feeling better with a list of exercises, and strangely enough, a new motto of sorts. I asked Bill at what point do I stop doing something; like standing, exercising, stretching; and he said. "Stop when it hurts."